Hs 40
HS 40 Bore is listed as 2.625, piston dia at bottom skirt is 2.62 inches. I'd think it is an HS40 or newer LAV40 or TVS 105.. The HS40 has a round exhaust port on the right, smaller similar engines had exhaust on the left or side exhaust. The HS50 was similar to the HS40, but part of the intake and exhaust ports were opened up larger. Just the head, tin work etc makes $20 seem a good deal. Shipping cost is always the bummer.